1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography, e.g., a copier, a printer, a facsimile machine, or a combination machine thereof.
2. Description of the Related Art
An image forming apparatus employing electrophotography fuses, by heat, an unfixed toner image formed on paper to fix the same thereon. Such an image forming apparatus is therefore provided with a heater as a heat source. When the heater is supplied with power, through up-regulation and through down-regulation are used to prevent a power supply voltage from dropping temporarily due to inrush currents. The through up-regulation is a control method in which power to be supplied to the heater is gradually increased with the lapse of time in an early stage where the heater is energized. The through down-regulation is a control method in which power to be supplied to the heater is gradually decreased with the lapse of time in a late stage where the heater is energized.
As power control methods such as the through up-regulation and the through down-regulation, control methods, e.g., phase control and wave number control, are used. FIG. 11 is a diagram showing a voltage waveform by using which the phase control is described. FIG. 12 is a diagram showing a voltage waveform by using which the wave number control is described.
The phase control is a control method for making power to be supplied to a load variable. To be specific, an AC power source supplies AC power to the load within a range of an arbitral phase angle in a half cycle to a zero-crossing point, and the arbitral phase angle is changed from one to another to make the power to be supplied to the load variable. For example, a voltage having the waveform shown in FIG. 11 is supposed to be applied to a heater as the load. In the illustrated example of FIG. 11, a half cycle corresponds to one control period. As shown in FIG. 11, as the voltage to be supplied per one control period is increased with the lapse of time, power to be supplied to the heater is also increased gradually.
The wave number control is another control method for making power to be supplied to a load variable. To be specific, a half cycle of AC power is regarded as one unit, and power to be supplied to the load is variable by turning ON or OFF for each half cycle. For example, a voltage having the waveform shown in FIG. 12 is supposed to be applied to a heater as the load. In the illustrated example of FIG. 12, four cycles correspond to one control period. As shown in FIG. 12, the amount of voltage to be supplied to the heater per one control period is 50%.
The phase control or the wave number control has been conventionally in use for power control. A combined method of the phase control and the wave number control has been also used. For example, such a combined method is disclosed in Japanese Laid-open Patent Publication No. 2010-286649. The phase control and the wave number control, however, suffer from the following drawbacks. In the phase control, a heater is turned ON or OFF during a half cycle of AC power. The phase control, therefore, presents a problem that a harmonic current distortion or switching noise occurs due to rapid changes in current caused when the heater is turned ON. The wave number control can reduce the harmonic current distortion or the switching noise. In the wave number control, however, voltage variations in an AC power source may be larger than that in the phase control, and flicker is likely to occur.
There is also provided a power control device using a boost chopper. Unfortunately, such a device is expensive.
In view of this, more preferable power control devices are desired, and some of such devices are actually proposed. For example, the following power control device is disclosed in Japanese Laid-open Patent Publication No. 2010-186218. The power control device includes two heaters driven by AC power, and a secondary electrical member such as a motor driven by DC power. When only any one of the heaters is supplied with power instead of both the heaters supplied with power, the secondary electrical member is supplied with power.
In the power control device, power supplied from an AC power source can be distributed to the individual heaters and the secondary electrical member, and the power factor can be improved. Therefore, the power can be used effectively in the power control device.
However, an image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2010-186218 is not capable of supplying power to the two heaters and the secondary electrical member at one time. For example, while supplying power to one of the heaters, the image forming apparatus cannot supply power to the other heater and the secondary electrical member. Thus, the image forming apparatus has a low degree of flexibility in amount of power supplied to each load. The image forming apparatus, therefore, has a difficulty in supplying power to each load appropriately.